A multi-stage gearshift mechanism of an automatic transmission includes a plurality of planetary gearsets. A powertrain having such a plurality of planetary gearsets varies the torque in multi-stages and outputs it to an output shaft when receiving a converted engine torque from a torque converter. The more speeds the powertrain of an automatic transmission has, the better power performance and fuel consumption. Therefore, it is desirable to have as many speeds as possible in powertrains.
Even for the same number of speeds, durability, power transmission efficiency, and size/weight of a transmission are substantially dependent on how planetary gearsets are arranged. Therefore, research for more structural strength, less power loss, and more compact packaging are under continuing investigation.
Usually, development of a powertrain using planetary gearsets does not devise a wholly new type of planetary gearsets. To the contrary, it invokes how single/double pinion planetary gearsets are combined, and how clutches, brakes, and one-way clutches are disposed to the combination of planetary gearsets such that required shift speeds and speed ratios are realized with minimal power loss.
As for a manual transmission, too many speeds cause a driver the inconvenience of excessive manual shifting. However, for an automatic transmission, a transmission control unit automatically executes shifting by controlling the operation of the power train, and therefore, more speeds usually implies more merits.
Accordingly, research of four-speed and five-speed powertrains has been undertaken, and recently, a powertrain of an automatic transmission enabling six forward speeds and one reverse speed has been developed.
An example of a conventional powertrain is shown in FIG. 14. The exemplary powertrain includes a combination of one simple planetary gearset SPG in the front and one Ravingneaux planetary gearset LPG of a Ravingneaux type in the rear. A first sun gear S1 of the simple planetary gearset SPG is fixed to a transmission case 1, and a second ring gear R2 (or equivalently, a third ring gear R3) of the Ravingneaux planetary gearset LPG is connected to an output gear OUT such that it acts as an output element
In addition, a first ring gear R1 of the simple planetary gearset SPG is fixedly connected to an input shaft 3, and a third planet carrier PC3 interconnecting second and third planetary gears P2 and P3 of the Ravingneaux planetary gearset LPG is variably connected to the input shaft 3 interposing a second clutch C2.
In addition, a first planet carrier PC1 carrying first planetary gear P1 of the simple planetary gearset SPG is variably connected to a third sun gear S3 of the Ravingneaux planetary gearset LPG interposing a first clutch C1. In addition, the first planet carrier PC1 is variably connected to a second sun gear S2 interposing a third clutch C3.
The second sun gear S2 is connected to the transmission case 1 interposing a first brake B1. A third planet carrier PC3 carrying second and third planetary gears P2 and P3 of the Ravingneaux planetary gearset LPG is connected to the transmission case 1 interposing a second brake B2 and a one-way clutch OWC in parallel.
Such a powertrain is operated as shown in FIG. 15 to realize six forward speeds and one reverse speed. That is, the first clutch C1 and the one-way clutch OWC (or equivalently the second brake B2) operate in a first forward speed, the first clutch C1 and the first brake B1 operate in a second forward speed, the first clutch C1 and the third clutch C3 operate in a third speed, the first clutch C1 and the second clutch C2 operate in a fourth speed, the second and third clutches C2 and C3 operate in a fifth speed, the second clutch C2 and the first brake B1 operate in a sixth speed, and the third clutch C3 and the second brake B2 operate in a reverse speed.
According to such a powertrain, as shown in FIG. 16, three planetary gearsets are under a load for power transmission at one speed, i.e., at a second forward speed. At three speeds, i.e., at first, fourth, and fifth speeds, two planetary gearsets are under a load for power transmission. At two speeds, i.e., at third and sixth speeds, one planetary gearset is under a load for power transmission.
Therefore, at any speed, at least one planetary gearset becomes under a load of power transmission, and accordingly, power transmission efficiency becomes deteriorated.
According to such a powertrain, the third sun gear S3 of the Ravingneaux planetary gearset LPG is connected to one operational element of the simple planetary gearset SPG, and accordingly, it is under a high load amounting about 1.5 times of input torque. Therefore, durability of a powertrain is deteriorated.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known in this country to a person of ordinary skill in the art.